Hard material layer

ABSTRACT

A hard material layer having a relatively low frictional resistance, which can be produced by impregnating the layer with a lubricant. Zirconium oxynitride layer, which has zirconium-, nitrogen- and oxygen-containing phase, is used as a hard material layer. The presence of the phase in the zirconium oxynitride layer leads to a perceptible reduction of the layer frictional resistance, such that further processing steps can advantageously be saved on coating of substrates, such a impregnation. The coating ( 26 ) can, for example, be applied on a tool ( 27 ), which is appropriate for metal-cutting machining a work piece ( 28 ). With the low frictional resistance, a dry machining of the work piece ( 28 ) can be executed with the tool. Another application is the coating of highly stressed components of fuel injection valve.

The invention relates to a hard material layer with a relatively lowfrictional resistance.

Hard material layers of this type are disclosed by German Patent DE 4418 517 C1. According to this document, it is possible for awear-resistant layer consisting of hard materials to be applied tosurfaces which are subjected to abrasive loads; on account of itshardness, this wear-resistant layer contributes to increasing theresistance to wear.

However, according to DE 44 18 517 C1, the wear-resistant layer has arelatively high frictional resistance. This can be reduced byimpregnating the wear-resistant layer which is formed with suitablelubricants in a further process step carried out in vacuo under theapplication of pressure. In this step, by way of example inorganiclubricants which are able to withstand high temperatures and reduce thefrictional resistance of the surface of the hard material layer areintroduced into the latter.

The object of the invention is to provide a hard material layer with arelatively low frictional resistance which is relatively simple toproduce.

According to the invention, the hard material layer is a zirconiumoxynitride layer which includes a phase which contains zirconium,nitrogen and oxygen.

Although the dissertation “Röntgenstrukturuntersuchungen anOxinitridschichten auf Chrom- und Zirkoniumbasis”, [X-ray structureanalyses of oxynitride layers with a chromium and zirconium base] byStéphane Collard, which was written at the Technical University ofChemnitz, Germany, has disclosed the production of oxynitride layerswith a chromium and zirconium base with the aid of a reactive sputteringprocess, including a layer with a phase which contains zirconium,nitrogen and oxygen, this dissertation does not give any indication thata layer with a phase which contains zirconium, nitrogen and oxygen isdistinguished not only by hardness but also by a low frictionalresistance and therefore—unlike in the prior art dealt with in theintroduction—can be produced in a single process step without anyaftertreatment.

The invention is based on the discovery that the phase which containszirconium, nitrogen and oxygen is primarily responsible for asurprisingly low frictional resistance of the zirconium oxynitride layeraccording to the invention compared to other hard material layers, inparticular also other zirconium oxynitride layers which do not containsaid phase. The zirconium oxynitride layer according to the inventiondoes not have to be formed exclusively by this phase, but rather thelayer composition may also contain phases of zirconium oxide orzirconium nitride.

The layers according to the invention, on account of their high hardnessand their low frictional resistance, provide excellent wear protectionto components which are subject to high loads. These layers can beproduced by sputtering in a single process step, which means that theyare very simple to produce. In particular, further treatment steps aimedat reducing the frictional resistance of the layer formed, for exampleimpregnation with a lubricant, can be dispensed with. This results in agood cost-performance ratio when the zirconium oxynitride layeraccording to the invention is used in terms of the protection againstwear which can be achieved and the production costs which are incurred.

According to one configuration of the invention, the phase whichcontains zirconium, nitrogen and oxygen has a defined crystal structure.The defined crystal structure of this phase makes an advantageouscontribution to the abovementioned reduction in the frictionalresistance of the hard material layer which includes this phase.

According to a further configuration of the invention, the phase of thezirconium oxynitride layer which contains zirconium, nitrogen and oxygenhas a cubic crystal structure with a unit cell parameter a of between0.511 nm and 0.532 nm. For this defined crystal structure, it has beenfound that the frictional resistance of the zirconium oxynitride layerwhich contains this phase is particularly low, so that this layer isparticularly suitable for use for the coating of components which aresubject to wear.

The zirconium oxynitride layer according to the invention can beproduced, for example, by means of a reactive sputtering process.Production conditions which in this case lead, for example, to theformation of phases which contain zirconium, nitrogen and oxygen andhave a defined crystal structure in a zirconium oxynitride layer arelisted in the annex to the dissertation cited above. Accordingly, thereactive sputtering can be carried out using a zirconium target, withthe distance between target and substrate to be coated being selected tobe 6.5 cm. The production temperature is 400° C., with a dischargecurrent of 1.5 A being maintained. Nitrogen and oxygen are supplied tothe sputtering process, with the nitrogen flow rate being set at 2.8sccm (standard cubic centimeters per minute), and it being possible forthe oxygen flow rate to be varied between 0.25 and 2 sccm. Whereas atlow and high oxygen flow rates only the formation of zirconiumoxynitride layers with phases which contain zirconium and nitrogenand/or with phases which contain zirconium and oxygen is observed, thezirconium oxynitride layers according to the invention can be producedwith a phase which contains zirconium, nitrogen and oxygen, and inparticular has a defined crystal structure, at medium oxygen flow rates.By way of example, at oxygen flow rates of between 0.5 and 1 sccm, it isalso possible for a phase of this type to be formed with a cubicstructure and a unit cell parameter a of between 0.511 nm and 0.532 nm.The nitrogen content in the phase which contains zirconium, nitrogen andoxygen and has a defined structure may be between 30 and 44 atomicpercent, and the oxygen content may be between 18 and 39 atomic percent.

Different applications result for the zirconium oxynitride layeraccording to the invention.

For example, according to German Patent Application DE 199 44 977 A1, toachieve a required service life it is proposed that coatings of chromiumnitride or of carbon-containing tungsten carbide be applied to parts offuel injection valves which can move relative to one another, so thatthe resistance to wear can be increased.

The object of the invention is to provide a fuel injection valve whichhas in relative terms an extended service life.

The object is achieved by a zirconium oxynitride layer with a phasewhich contains zirconium, nitrogen and oxygen and in particular has adefined structure being used as hard material layer for at least one oftwo valve parts, which are in contact with one another and can moverelative to one another, in a fuel injection valve of an internalcombustion engine. This defined structure may, for example, be a cubicstructure with a unit cell parameter a of between 0.511 nm and 0.532 nm.This advantageously makes it possible to greatly reduce the wear on thecoated component and also on the uncoated partner. A further improvementcan be achieved if both components which move relative to one anotherare coated. The fuel which is delivered means that the lubrication stateis less than optimum while the components are moving, since the fuel perse does not have optimum lubrication properties. However, it is notpossible to add an additive in order to improve the lubricationproperties, on account of the subsequent combustion in the internalcombustion engine. Therefore, the layer structure according to theinvention represents a solution for improving the wearing properties ofthe coated valve components. The reduction in the wear between thecomponents which move relative to one another makes it possible tolengthen the surface life of the injection valve.

An advantageous embodiment of the invention is achieved if, with thefirst valve part being a valve seat having a valve needle and the secondvalve part having an injection opening which can be closed off by thevalve needle, at least the valve needle is provided with the zirconiumoxynitride layer according to the invention. The valve needle is thecomponent which is subject to the highest loads in the injection valveand is therefore considered critical for the service life of the valve.

It is regarded as advantageous for the walls of the injection nozzlelikewise to be coated. This makes it possible to further increase theexpected service life of the fuel injection valve.

Furthermore, coated tools are known, for example from U.S. Pat. No.6,284,356 B1. To extend the service life of the tool, the layers areapplied to surfaces of the tool which are subject to high levels ofload. They therefore serve as wear-resistant layers.

A further object of the invention is to provide a tool which bycomparison has a longer service life.

This object is achieved by a zirconium oxynitride layer with a phasewhich contains zirconium, nitrogen and oxygen and in particular has adefined crystal structure being used as a surface layer on the tool.This advantageously allows wear to the tool to be reduced underfrictional loading. This is achieved by virtue of the special propertiesof the phase which contains zirconium, oxygen and nitrogen and ispresent in the zirconium oxynitride layer, since this phasesignificantly reduces the frictional resistance of the layer compared toconventional tool coatings. A defined crystal structure may inparticular be a cubic structure with a unit cell parameter a of between0.511 nm and 0.532 nm. This opens up the possibility of tools which areprovided with the zirconium oxynitride layer according to the inventionbeing used for the dry machining of workpieces. Dry machining is to beunderstood as meaning machining of workpieces without the assistance oflubricants, in which case the dry lubrication properties of the layeraccording to the invention can be exploited. The elimination of coolantsin, for example, material-removing machining of workpieces represents anenormous potential saving, since correspondingly designed machine toolsmake do without a lubricant circuit and the associated procurement andoperating costs. At the same time, the environmental pollution caused bylubricants can be avoided, which not least is also of benefit to theeconomics of dry machining of workpieces, since the disposal of thelubricants incurs costs.

The layer according to the invention can advantageously be applied notonly to tools for material-removing machining, but also to tools forchipless machining. Tools of the latter type are exposed to highfrictional loads during the machining of the workpiece. The resultantwear can be beneficially reduced by the layers according to theinvention. One example of a chipless machining process is fluting, whichis used, for example, for thread forming.

The low frictional resistance of the zirconium oxynitride layeraccording to the invention opens up the possibility of using it asprotection against wear for components which can be provided with littleif any lubrication in use. In applications of this type, the absence orinsufficient action of a lubricant can be compensated for by theexpedient frictional properties of the layer.

This reduces the load not only on the coated component but also on africtional partner which interacts with this component, irrespective ofwhether or not this frictional partner itself has any protection againstwear.

Further details of the invention are to be found in the drawing, butthese do not constitute any restriction to the claimed invention. In thedrawing:

FIG. 1 diagrammatically depicts an exemplary embodiment of a fuelinjection valve of an internal combustion engine, taken centrally insection, and

FIG. 2 diagrammatically depicts an exemplary embodiment of a tool in theform of a turning tool.

FIG. 1 illustrates a fuel injection valve which can be installed in acylinder head (not shown), in such a manner that it has an injectionopening 11 opening out into the combustion chamber (likewise not shown)of the cylinder. The fuel injection valve, also comprises, as actuator,a magnet coil 12 which is encapsulated in a coil housing 13, a tubularinner pole 14 and a sleeve-like outer pole 15, which opens out into anozzle body 16.

A valve seat body 17 which forms the injection opening 11 is introducedinto the nozzle body 16. The valve is closed off by means of a valveneedle 18, which can open and close the injection opening 11 by means ofa seat surface 19. For this propose, the valve needle is secured to anarmature 20 which the magnet coil 12 can move, counter to a closureforce exerted by a spring 21, away from the valve seat toward an openposition (not shown). In the open position, the fuel can be guidedthrough a passage system 22, in accordance with the arrows indicated,the fuel being supplied to the injection opening 11 through a feed bore23, an installation space 24 for the spring 21, a groove 25 in thearmature 20 and the nozzle body 2.

To improve the resistance to wear, the valve needle 18 and the valveseat body 17 are provided with a zirconium oxynitride layer whichincludes a phase which contains zirconium, nitrogen and oxygen and has adefined structure (not shown in more detail). These two parts may becoated as individual parts and then subsequently fitted to theassociated components of the injection valve. The layer is of particularimportance in the region of the highly loaded valve seat surface 19; inthis area, particularly thick layers can be achieved by suitablepositioning of the components in the sputtering diode. However, it isalso possible for the adjoining areas, i.e. the injection opening 11 inthe valve seat body 17 and the stem of the valve needle 18 to be coated.

A further example of an application for the use of a zirconiumoxynitride layer 26 is illustrated in FIG. 2. This is a tool 27 for thematerial-removing machining of a workpiece 28. The tool 27 is secured ina chuck 29, while the workpiece 28 rotates in the direction indicated bythe arrow. This is therefore a simplified representation of themachining of the workpiece 28 by turning. The tool 27 peels off a chip31 along a cutting edge 30, this chip shearing over part of the surfaceof the tool 27. No lubricants are used, but rather the zirconiumoxynitride layer 26 has a phase which contains zirconium, nitrogen andoxygen and has a defined structure, so that it presents only a lowfrictional resistance to the chip 31. This allows dry lubrication.

1-7 (cancelled)
 8. A hard material layer with a relatively lowfrictional resistance, the hard material layer comprising a zirconiumoxynitride layer which includes a phase which contains zirconium,nitrogen and oxygen and has a cubic structure with a unit cell parameterbetween 0.511 nm and 0.532 nm.
 9. Method of increasing the service lifeof at least one of two valve parts, which are in contact with oneanother and can move relative to one another, in a fuel injection valveof an internal combustion engine, which comprises applying a hardmaterial layer comprising a zirconium oxynitride layer with a phasewhich contains zirconium, nitrogen and oxygen and has a cubic structurewith a unit cell parameter between 0.511 nm and 0.532 nm to said atleast one of two valve parts.
 10. The method as claimed in claim 9,wherein the first valve part is a valve seat with a valve needle, thesecond valve part has an injection opening which can be closed off bythe valve needle, and the hard material layer is applied to the valveneedle.
 11. The method as claimed in claim 10, wherein the injectionopening has walls, and the hard material layer is applied to said wallsof the injection opening.
 12. Method of extending the service life of atool, which comprises applying a zirconium oxynitride layer having aphase which contains zirconium, nitrogen and oxygen and has a cubicstructure with a unit cell parameter between 0.511 nm and 0.532 nm as asurface layer on said tool.